Two similar plano convex lens are placed with their convex surfaces in contact and the space between them is filled with a liquid of refractive index 1.7, as shown in figure. The focal length of plano convex lens is 25 cm and radius of

curvature of the curved surface is 12.5 cm and is made of glass of refractive index 1.5. Find the focal length of the combination.

nope the ans is wrong....the given ans is -31.25 but i am getting -8.29

See buddy....what I feel is........

this can be considered as a case of lenses in contact with each other.......

here we have...2 similar plano-convex lenses kept in opposite directions to each other........so their powers will simply cancel each others effect and we only need to consider one concave lens made out of liquid...kept in medium of glass....whose focal length will give us the focal length of the combination.....

that was the concept I used.....If u find any mistake then pls let me know>>>>>

yup u r perfectly correct thats wat i also did

1/f=(1.7-1)(-1/12.5-1/12.5)

1/f=-0.112

f=-8.29cm

But hey........bro......u also have to consider the refractive index of glass.........

I mean to say.....while calculating the focal length of the concave lens.......the value of refractive index shud be

.......i.e

.....

I think we have to do it this way/////////

ya i think you r right i dunno how cud i go wrong with this ...... so do u say the given ans is wrong ?????

ya i too think so.........anyways thanx 4 helping me out

When we say P=P1+P2+...  we are using  the superposition principle which means we have consider them separately like in air then add them so i don't think here we need to do 1.2/1.5 it would as in air which is 1.7/1..so proceed .

well this is getting more and more confusing for me..........I think risin is correct........

thanks!

no i dont agree wid resin see guys refractive index is always comparitive i.e with respect to other media so how is it possible to compare it wid air then add up????really confused

Now it is upto you..had this same doubt...only den the prof remimded that we are using super position principle,which means we have to take separately...was kinda of embarrased in class..so remember this well.